From a user point of view, volume visualization allows the user to quickly inspect the volumetric image data set and to select a region of interest to the user, such as a radiologist. The algorithms like maximum intensity projection (MIP), closest vessel projection (CVP) and average intensity projection (AIP) are based on ray casting. For each pixel of the displayed image a ray is cast from said pixel through the image data volume, typically a slab. The intensity of a pixel is computed based on the intensities of voxels along the cast ray. For MIP, the intensity of the pixel is the intensity of a voxel along the cast ray having the highest intensity. For CVP, the intensity of the pixel is the intensity of a first voxel along the cast ray having a maximum intensity, possibly a local maximum intensity, greater than a CVP threshold. For AIP, the intensity of the pixel is the average of intensities of voxels along the cast ray. However, the information comprised in these images does not take into account the location of pixels along the cast ray.
Recently, a new method called volume intensity projection (VIP) was introduced by Philips Medical Systems. The idea of VIP is that intensities of voxels along a ray cast from a pixel are multiplied by weight factors. The larger the distance from a voxel to said pixel, the smaller is a weight factor. The voxels with weighed intensities are then used to compute an image, using e.g. MIP.
The current rendering techniques have their advantages and disadvantages. For instance, suppose that a volume comprises a lot of vessels close to each other with slightly different brightness. When applying the VIP technique to this case, vessels that are further away from a viewing plane appear darker than vessels that are closer to the viewing plane, which may give radiologists the false impression that vessels that are further away from a viewing plane have less blood flow than vessels that are closer to the viewing plane. On the other hand, suppose that a volume comprises the aorta on top of the vertebra, both the aorta and the vertebra having similar intensities. Applying the VIP technique to this case advantageously results in the aorta appearing brighter than the vertebra. Other rendering techniques may also be useful for visualizing volumetric image data sets.